Introduction and MapReduce - SNAP - Stanford University

M map tasks, R reduce tasks
Rule of a thumb:
Make M and R much larger than the number of
nodes in cluster
One DFS chunk per map is common
Improves dynamic load balancing and speeds
recovery from worker failure
Usually R is smaller than M
because output is spread across R files
1/8/2012 Jure Leskovec, Stanford CS246: Mining Massive Datasets, http://cs246.stanford.edu 47